Polyesters, namely poly(ethylene terephthalate) or PET, are widely used in the food package industry. The PET films compete commercially with multilayer structures consisting of ethylene vinyl alcohol copolymer (EVOH), polyvinylidene chloride (PVDC), or nylon were commonly used to produce films with reduced gas permeability. The multilayer films have a number of disadvantages. Films of PVDC require special handling during processing, and reprocess poorly. The barrier properties of films of EVOH degrade significantly with moisture.
In developing PET for food package industry applications, a major effort has been directed to reducing the gas permeability of the package, since a decrease in such permeability will lead to a longer shelf life of the food product. In addition, the dimensional stability and the heat stability of the film are relevant to storage at ambient temperature over long periods of time or upon heating (with contents) the contents in either microwave or conventional ovens. Equally important considerations and development(s) relate to the breakage of seals and warping due to excessive shrinkage of the container during retort or any of the thermal sterilization processes. The versatility of PET is, in part, attributable to its potential for crystallization. Amorphous polymers are optically clear, but because they are glassy in nature, they tend to lack the necessary ductility and toughness. A semi-crystalline polymer, while often very tough, is not optically clear because of the formation of large, light diffracting spherulite-type crystals.
Crystallization of PET occurs when the PET is cooled below T.sub.m [its melting temperature]. The thermodynamic driving force for nucleation increases as the polymer is cooled below the melting point. Thermal crystallization occurs with decrease in temperature unless the cooling rate is fast enough. Thermal crystallization produces spherulites, which are large, roughly spherical crystal superstructures composed of many crystal lamellae growing radially from the center. Because the size of spherulites is comparable to the wavelength of light, they scatter light and make the polymer look hazy, possibly even opaque. Once thermal cryallization occurs, it becomes much more difficult to stretch/form the polymer to any significant degree.
Stretching and orientation also produce crystals and crystallinity in PET. However, the crystals of stretching and/or orientation are not large round spherulites that form; rather, on stretching and/or orientation, numerous, very small oriented crystals result. Suc oriented crystals are too small to scatter light so a film/fiber remains clear. The resultant polymer tends to be very strong and tough.
Intrinsic viscosity, IV, a direct function of molecular weight, can affect crystallization. Higher IV results in a slower crystallization rate, and higher toughness. High melt viscosities, resulting from relatively higher molecular weight, can mean increased difficulties in manufacture and downstream processes.
Solution or intrinsic viscosity (IV) is a property which characterizes the polyesters. The technique for measuring IV involves measuring the time it takes for different concentrations of polymer in a solvent, usually 60/40 phenol/tetrachlorethane to pass through a viscometer relative to the flow time of pure solvent. It can be shown for PET that IV is directly related to the weight-averaged molecular weight (M.sub.w).
The blown bubble extrusion process has been used in the past for producing PET film. This process utilizes a tube which is extruded from an annular die and then inflated with air to a size dictated by the film properties desired and equipment configuration. The polymer (e.g., PET) is then cooled with blown air or cascading water, and can be collapsed into a flat tube. The film is then wound into rolls of either slit or tubular film. As described, this single bubble blown film process moderately stretches and/or orients the PET. The PET must be melted for extrusion. For the film sheet industry melting is traditionally performed using single screw extruder. Polymer in the form of either pellets or powder, is fed into the feed hopper from dryers, where it is conveyed, melted and pumped to the die by the rotating action of a screw or multiple screws in the case of twin screw extrusion. PET resin is typically processed with barrel temperatures between 260 degrees C. and 300 degrees C. Higher IV resins require higher initial temperatures to compensate for the increased viscosities and to minimize torque requirements and die pressures. The conventional extruder should have an L/D (length /diameter) ratio of at least 24:1. Blown film processes involving only a single bubble, as is typical with polyethylene film production, result in minimal orientation. For double-bubble blown film air pressure is used to inflate a second bubble to orient the polymer [orientation in the first bubble is minimal since forming temperatures are too high].
The PET can be fed to the extruder in the blown film line as pellets. The pellets are produced utilizing conventional methods by melting the PET in a melt reactor from which it emerges as a low molecular weight resin. Solid state polymerization can be used to increase the molecular weight. For example, prior to pellet formation, the polyester can undergo a solid state polymerization, in tower reactors to increase IV, intrinsic viscosity, and/or molecular weight. The targets for commercial products, include IV of 0.75 to 0.85 for bottle applications; and IV of 0.85 to 1.1 is required for sheet product(s), thermoformed into shaped objects. The melt is then cooled through a water bath and processed into pellets. The rapid cooling of the melt creates an amorphous structure. Surface crystallization is needed prior to drying the polymer, because amorphous PET pellets will stick to each other during the drying processes. Prior to extrusion PET supplied as pellets or powder must be dried. The higher the IV, or the molecular weight, the more critical is the need for drying, as water leads to a significant drop in molecular weight as well as causing bubbles or voids in the final product.
To summarize the stages of production of PET film, pellets of PET are formed and crystallized on the surface; the pellets are extruded to form film; the film is oriented uniaxially or biaxially; the film can be heat set and crystallized.